If I watch a long string of episode I'll listen to an intro once and not listen again until the next time I come back after short period break of like a day1/2 - 1day. The same goes for "outtros". Once I decide that i'm done for a bit. I listen to the outtro. The intro and outtro are my begin and end to most anime unless the song is terrible. Theme song have also driven my fandom of Japanese music in general. Most of all popular artist will high record sales have done an intro or outtro to an anime. There has only been one that I know that hasn't done one at all: Ayumi Hamasaki. I guess she's so good that she doesn't need the publicity. Her sales world probably shot 10 fold if she did.